This is the 10297th most frequent Russian word.
бревну
"Бревну" is the dative case of "бревно," meaning "log" or "timber" (to/for the log).
Here, "бревно" is used in its literal meaning, denoting a piece of wood.
Он сел на бревно, чтобы отдохнуть.
He sat on the log to rest.
Here, "бревно" is used metaphorically to describe something simple or plain.
В шахматах пешка проста, как бревно.
In chess, the pawn is simple like a log.
In this figurative usage, "бревно" denotes passivity or inaction, contrasting desired involvement.